This invention relates generally to polyether polyols formed using aluminum phosphonate catalysts and, more particularly, to formation of polyetherols having very low unsaturation using aluminum phosphonate catalysts.
Polyoxyalkylene polyether polyols are well known compounds utilized in the formation of a variety of polyurethane products, such as foams, coatings, adhesives, sealants and elastomers. As a general matter, these polyols are produced by polyoxyalkylation of an initiator molecule with ethylene oxide, propylene oxide, butylene oxides, or mixtures thereof. The initiator molecules contain alkylene oxide-reactive hydrogens like hydroxyls and amines. This oxyalkylation is generally conducted in the presence of a catalyst. The most common catalysts are basic metal catalysts such as sodium hydroxide, potassium hydroxide, or alkali metal alkoxides. One advantage of these base catalysts is that they are inexpensive and readily available. Use of these base catalysts, however, is associated with a range of problems. One of the major problems is that the oxyalkylation with propylene oxide has associated with it a competing rearrangement of the propylene oxide into allyl alcohol, which continually introduces a monohydroxyl-functional molecule. This monohydroxyl-functional molecule is also capable of being oxyalkylated. In addition, it can act as a chain terminator during the reaction with isocyanates to produce the final urethane product. Thus, as the oxyalkylation reaction is continued more of this product, generally measured as the unsaturation content of the polyol, is formed. This leads to reduced functionality of the polyol and a broadening of the molecular weight distribution of the final polyol mixture. The amount of unsaturation content may approach 30 to 40% with unsaturation levels of 0.08 meq/g KOH or higher.
In an attempt to reduce the unsaturation content of polyols a number of other catalysts have been developed. One such group of catalysts includes the hydroxides formed from rubidium, cesium, barium, and strontium. These catalysts also present a number of problems. The catalysts only slightly reduce the degree of unsaturation, are much more expensive, and some of them are toxic. Like potassium hydroxide catalysts, these higher molecular weight hydroxide catalysts are known to affect the polyurethane forming reaction, they are generally removed prior to work-up of any polyol for use in polyurethane systems.
A second line of alternative catalyst development has been formation of double metal cyanide (DMC) catalysts. These catalysts are typically based on zinc hexacyanocobaltate. With the use of DMC catalysts it is possible to achieve unsaturations in the range of 0.003 to 0.010 meq/g KOH. While the DMC catalysts would seem to be highly beneficial they also are associated with a number of difficulties. As a first difficulty there is a relatively high capital cost involved in scaling up of and utilization of DMC catalysts. The catalysts themselves have an extremely high cost compared to the base catalysts. The process of making polyols using DMC is also different from based catalyzed reactions. During use of DMC catalysts there is an initial significant, and often unpredictable, lag time before the catalyst begins catalyzing the reaction. Another difficulty is that ethylene oxide does not add uniformly to growing polymer chains utilizing DMC catalysts. Chain transfer is slow relative to chain growth, so all the ethylene oxide adds to only a few of the polymer chains, leaving the rest unreacted. The result is a polyol of such low quality that it has no commercial value. To add ethylene oxide to a growing chain the DMC catalysts must be replaced with the typical base catalysts, thus adding steps. In addition, it is generally believed that the DMC catalysts should be removed prior to work-up of any polyol for use in polyurethane systems. Finally, polyols generated using DMC catalysts are not mere “drop in” replacements for similar size and functionality polyols produced using the typical base catalysts. Indeed, it has been found that often DMC catalyzed polyols have properties very different from equivalent polyols produced using, for example, potassium hydroxide. It is recognized in the art that polyols made utilizing DMC catalysts contain small amounts of high molecular weight compounds which can affect utilization of these polyols in polyurethane systems, particularly foaming. The so-called high molecular weight tail has been identified in amounts of greater than 100 ppm and variously described as polymer of molecular weight greater than 50,000 Daltons, see U.S. Pat. No. 5,919,988 which is incorporated herein by reference. The presence of the so-called high molecular weight tail in amounts of greater than 300 has been identified as a cause of foam destabilization and collapse.
Thus, there exists a need for a class of catalysts that can be used for the oxyalkylation of initiator molecules by alkylene oxides that is inexpensive, capable of producing very low unsaturation polyols, does not require removal from the polyol prior to utilization in polyurethane systems, and that produces a polyol having properties that are the same or better than those in a polyol produced using base catalysts. Preferably the new class of catalysts can be used in existing systems and equipment using standard manufacturing conditions.